Privacy advocates have asked US regulators to halt the $19 billion Facebook acquisition of messaging service WhatsApp.

They want a clearer understanding of how the company intends to use the personal data of WhatsApp's 450 million users.

WhatsApp, a service that allows mobile phone users to send each other messages, has had a longstanding commitment to not collect user data for advertising purposes.

But there's no guarantee that that commitment will hold true once the service becomes part of Facebook, according to the filing to the Federal Trade Commission by the Electronic Privacy Information Center and the Center for Digital Democracy, both non-profit groups.

The complaint asks regulators to investigate the deal "specifically with regard to the ability of Facebook to access WhatsApp's store of user mobile phone numbers and metadata."

Facebook, the world's largest social network with 1.2 billion users, generates the majority of its revenue by showing ads that target users by age, gender and other traits.